ABS resin is an acrylonitrile-butadiene-styrene terpolymer and has excellent impact resistance, rigidity, chemical resistance, and processability, and thus is used for various purposes in various fields such as electronics, construction, and automobiles. However, since ABS resin is prepared using a butadiene rubber polymer, ABS resin has a low weatherability and is not suitable as an outdoor material.
Therefore, to prepare a thermoplastic resin having excellent physical properties, weatherability, and aging resistance, ethylene-based unsaturated polymers associated with aging caused by ultraviolet light should not be present in graft copolymers. As such a resin, acrylonitrile-styrene-acrylate (ASA) resins using crosslinked alkyl acrylate rubber polymers are typical. These ASA resins have excellent weatherability and aging resistance and are used in various fields such as automobiles, ships, leisure goods, building materials, and gardening.
A method of preparing an ASA polymer having weatherability and aging resistance was disclosed in German Patent No. 1,260,135, and a core rubber used in the patent is a crosslinked large-diameter acrylate rubber latex having an average particle diameter of 150 to 800 nm and a narrow particle size distribution. This ASA polymer has improved notched impact strength, but is difficult to color, as compared with an ASA polymer prepared using a small-diameter polyacrylate latex. The difficulty in coloring results in a faded pastel color rather than a bright color, and thus the use thereof is limited in preparation of a colored molded article having high saturation.
U.S. Pat. Nos. 4,224,419 and 5,932,655 and European Patent No. 534,212 disclose a molding material using a mixture of a first graft copolymer containing a crosslinked alkyl acrylate rubber polymer having a small average particle diameter and a second graft copolymer containing a crosslinked alkyl acrylate polymer having a large average rubber particle diameter. This molding material has excellent weatherability and mechanical properties and improved coloring properties. However, since the refractive index of the crosslinked alkyl acrylate polymer contained in the molding material is very different from the refractive index of a grafted styrene-acrylonitrile copolymer, a lot of light scattering occurs and a clear color may not be realized when the molding material is colored.
To overcome these drawbacks, Japanese Unexamined Patent Application Publication Nos. 47-47863 and 59-49245, Japanese Patent Laid-Open Publication No. 3-66329, U.S. Pat. Nos. 4,912,162 and 4,393,172 and the like disclose a method of incorporating a diene-based rubber polymer inside an alkyl acrylate rubber polymer. However, this method has a limitation in preparing a resin having high weatherability because a diene-based polymer is introduced.
In addition, a method of introducing monomers that increase the refractive index of a polymer at the stage of preparing an alkyl acrylate polymer rubber is disclosed in U.S. Pat. No. 5,777,036 and U.S. Patent Publication No. 2007/0287799 and European Patent No. 904,305. However, this method has a limitation in selection of monomers and has a disadvantage that preparation cost is greatly increased.
To solve these problems, U.S. Pat. No. 6,720,386 and U.S. Patent Publication Nos. 2003/0236350 and 2006/0069208 disclose a terpolymer of alkyl methacrylate-aromatic vinyl compound-vinyl cyanide compound as a hard thermoplastic resin forming a matrix phase and an alkyl methacrylate polymer, and a method of lowering the refractive index of a matrix phase by using the same alone or in combination thereof. However, a weather-resistant resin prepared by the above-described method still has insufficient light transmittance and therefore is not satisfactory in coloring properties. Further, no method of improving the yield of the weather-resistant resin is described in the above method.